Download 08. pons + midbrain

Internal structure of pons
There are 3 levels of transverse
sections :
1-lower level : level of facial colliculus.
2-mid-level : level of trigeminal nerve.
3-upper level : level of 4 lemnisci.
The pons at the three levels is divided
into 2 main divisions :
1-Basilar part (Basis pontis) :
-it is the ventral part of pons,
-its contents are constant in all levels.
2-Tegmentum :
-it is the dorsal part of pons.
-its contents vary in the 3 levels of
pons. -Superiorly, the tegmentum of
pons is continous with tegmentum of
midbrain.-
.
Ventral (basilar) part of pons
in all levels: It is marked by numerous
transverse pontocerebellar fibres
which arise from pontine nuclei
buried in its substance.
These transverse Pontocerebellar
fibres cross the midline to pass to
contralateral cerebellar hemisphere
forming middle cerebellar peduncle
(brachium pontis), where trigeminal
N. (V) pierce it.
Mid-pons at level of
trigminal nerve.
Caudal pons at level
of facial colliculus
Ventral (Basilar)part of pons
in all levels:
Corticospinal fibres &
corticobulbar (pyramidal tract)
appear as small,separate
bundles running longitudinally
between fascicles of transverse
pontine fibres.
T.S of caudal pons at
level of facial
colliculus
T.S of mid-pons at level of trigeminal
T.S of rostral pons at
level of 4 lemnisci
Dorsal( tegmental) part of caudal pons at
level of facial colliculus :
1-Trapezoid body consists of
acoustic fibres responsible for hearing,
T.S of caudal
pons
arising from cochlear nuclei (dorsal &
ventral cochlear nuclei lying dorsal &
ventral to inferior crebellar peduncle in
the most lower part of pons).
-These acoustic fibres crossing opposite
side of pons forming decussation of
trapezoid body, then the axons ascend
into rostral pons & midbrain as lateral
lemniscus and terminate in inferior
colliculus.
-it lies in the anterior part of
tegmentum.
T.S of caudal
pons
T.S of rostral pons
Dorsal( tegmental) part of caudal pons at
level of facial colliculus :
2-Medial lemniscus :
T.S of caudal
pons
T.S of rostral pons
-(the most medial lemniscus) is
marking the boundary between
ventral & tegmental portions of
pons.
-It is a band of ascending
fibres carrying proprioceptive
& fine touch sensation from
opposite side of body, via
Gracile & Cuneate tracts,
ending in P.L.V.N. of thalamus.
Dorsal (tegmental)part of caudal pons at
level of facial colliculus :
3-Medial longitudinal
fasciculus or bundle :
-it is an association tract present
close to midline , found
throughout the brain stem,and
descend into spinal cord.
-It links vestibular nuclei with
motor ocular nuclei, supplying
extraocular muscles
(oculomotor,trochlear &
abducent nuclei) to serve
corrdination of head and eye
movements.
Dorsal (tegmental)part of caudal pons at
level of facial colliculus :
4-Abducent nerve nucleus :
site : in posterior aspect of caudal
pons near floor of 4th ventricle. It is
encircled by fibres of facial N.,
forming elevation in the the floor of
4th ventricle called facial colliculus.
-Its efferent fibres : pass
downwards traversing medial
lemniscus & pyramidal T.bundles to
emerge anteriorly at junction between
pons & medulla, supplying lateral
rectus muscle
T.S through caudal part of pons at
5-Facial motor nucleus :
the level of facial colliculus.
site : in posterior part of caudal pons.
Its efferent fibres encircle abducent
nucleus,then pass anterolaterally to
emerge at the junction between pons &
medulla, (supplying ms. of facial expression)
Dorsal (tegmental) part of caudal pons at
level of facial colliculus :
6-Vestibular nuclei : they are 4 nuclei lie
subjacent to vestibular area of floor of 4th
ventricle.
-they receive afferent fibres from the
vestibular nerve.
-they send efferent fibres as :
vestibulo-ocular fibres through
medial longitudinal fasciculus.
T.S through caudal part of
pons at the level of facial
colliculus.
7-Spinal tract & nucleus of trigeminal
nerve: lie on the anteromedial aspect of
inferior cerebellar peduncle.
-carrying pain & temperature sensations
from the face.
-The axons of the cells of the nucleus cross
to opposite side and ascend in trigeminal
lemniscus to end in thalamus.
8-Inferior cerebellar peduncle.
Dorsal (tegmental)part of mid-pons
at level of trigeminal nerve :
1-Trigeminal motor nucleus : medial
in position. its axons form the motor root
of trigeminal N. which passes along
mandibular N. (supplying muscles of
mustication).
T.S of mid-pons a level of trigeminal N. .
2-Trigeminal sensory nucleus : lateral
in position. It receives afferent touch &
pressure sensation from face.
-It sends efferent fibres which cross to
join trigeminal tract or lemniscus.
3-Superior cerebellar peduncle : lies
posterolateral to motor nucleus of
trigeminal nerve.
4-Medial longitudinal fasciculus,
spinal lemniscus , medial lemniscus
& trapezoid body.
Dorsal (tegmental) part of Rostral Pons at
level of 4 lemnisci :
1-Superior cerebellar peduncle :
lies in the rostral part of pons, forming
lateral walls of 4th ventricle.
-They are connected together by
superior medullary velum which forms
roof of 4th ventricle.
T.S of rostral pons
T.S of mid-pons at level
of trigeminal nerve.
Dorsal (tegmental) part of Rostral Pons at
level of 4 lemnisci :
Types of fibres in the S.C.P :
T.S of rostral pons
(A) Afferent fibres :
1-ventral spino-cerebellar tract : it
carries proprioceptive impulses
from the limbs to cerebellum.
2-tecto-cerebellar tract : it carries
auditory & visual impulses from
tectum of midbrain to cerebellum.
(B)Efferent fibres :
1-Dendato-rubral tract : it is
concerned with coordination of
movement. it ends in red nucleus
(Extrapyramidal nucleus) in
midbrain.
2-Dentato-thalamic tract : from
dentate nucleus of cerebellum to
end in ventral lateral nucleus of
thalamus.
Dorsal (tegmental) part of Rostral Pons at
level of 4 lemnisci :
2-Lateral lemniscus :
the most lateral lemniscus. it is a
band of ascending fibres carrying
hearing sensation from both ears
( mainly from opposite side), via
acoustic Fs. Of cochlear nuclei,
Ending in auditory area in
temporal lobe.
3-Spinal lemniscus : just
medial to lateral lemniscus. it is a
band of ascending Fs. Carrying
pain, tempreture & crude touch from
opposite side of body via
spinothalamic tract Ending in
P.L.V.N.of thalamus.
Dorsal (tegmental) part of Rostral Pons at
level of 4 lemnisci :
4-Trigeminal lemniscus: just
medial to spinal lemniscus.it is a
band of ascending Fs.carrying pain,
temp., touch & proprioception from
opposite side of face & scalp, via
sensory Fs. Of trigeminal N. ending
in P.M.V.N.of thalamus.
5-Medial lemniscus :
-(the most medial lemniscus) is
marking the boundary between
ventral & tegmental portions of pons.
- it is a band of ascending fibres
carrying proprioceptive & fine touch
sensation from opposite side of body,
via Gracile & Cuneate tracts, ending
in P.L.V.N. of thalamus.
Examples for Questions of Pons :
1. All of these tracts are found in the pons EXCEPT:
a.Corticospinal tract.
b.Corticobulbar tract.
c.Medial lemniscus.
d.Medial longitudinal fasiculus.
e.Gracile tract.
Pons :
2. Which nucleus is not lying in the tegmentum of pons ?
a. Facial motor nucleus.
b. Abducent motor nucleus.
c. Inferior olivary nucleus.
d. Cochlear nucleus.
e. Vestibular nucleus.
3. The acoustic fibres ascend as :
a. Medial lemniscus.
b. Spinal lemniscus.
c. Lateral lemniscus.
d. Trigeminal lemniscus.
e. Medial longitudinal tract.
Internal structure of Midbrain
The midbrain is divided into dorsal &
ventral portions at the level of cerebral
aqueduct.
(A)Tectum : the smaller dorsal part behind
aqueduct. It is composed of 4 rounded
swellings (colliculi) :
1-2 superior colliculi : lower centers of vision.
2-2 inferior colliculi : lower centers of hearing.
(B)2 Cerebral peduncles : the larger
ventral part in front of aqueduct. It consists of 3
parts :
1-Crus cerebri (Basis pedunculi) : the most
anterior part which consists entirely of
pyramidal & cortico-pontine fibres.
2-Substantia nigra : a thick lamina of grey
matter formed of deeply pigmented nerve cells
lying behind crus cerebri. It is an
Extrapyramidal motor centre.
3-Tegmentum : the post. part of cerebral
peduncle. It contains ascending tract,
decussation, nuclei, & reticular formation.
Caudal midbrain at level of
Inferior colliculus 1-The inferior colliculus is a centre
of hearing reflex which receives
ascending auditory pathway ,which
run in lateral lemniscus.
-Its Efferent Fs. end in medial
geniculate nucleus of thalamus,
which projects to auditory cortex of
temporal lobe.
s.
4-Mesencephalic
nucleus of Trigeminal :
-lies lateral to aqueduct of
midbrain, at level of
inferior&superior colliculi.
Sensory nucleus receives
proprioceptive sensation
from ms.of mastication.
2-The cerebral aqueduct runs
ventral to colliculi, and surrounding
by area of grey matter, the
peri-aqueductal (or central grey ).
3-Trochlear nucleus : lies ventral to
peri-aqueductal grey, its efferent Fs.
cross to opposite side to emerge
from back of midbrain, then turn
forwards to reach base of brain to
supply extraocular ms.(sup.oblique).
Auditory pathway & inferior colliculus
of midbrain (reflex center of hearing) :
Caudal midbrain at level of
Inferior colliculus
5-Medial longitudinal fasciculus : is a
well defined bundle of association
fibres lies on each side of median
plane in midbrain tegmentum.
-It extends throughout the brain stem,
and descends into spinal cord.
-It lies close to oculomotor, trochlear
& abducent nuclei.
-it receives fibres from vestibular
nuclei.
-it sends efferents to ocular motor
nuclei
-Its function : coordination of eye,and
head & neck movements.
Medial longitudinal fasiculus in
midbrain :
Caudal midbrain at level of
Inferior colliculus
6-Decussation of superior cerebellar
peduncles (brachium conjunctivum) :
the fibres of each peduncle cross to
opposite side, forming decussation in
the central part of tegmentum.
.
7-Medial lemniscus : it is a band of
ascending Fs. carrying proprioceptive
sensation from opposite side of body.
-It is the upward continuation of
gracile & cuneate tracts of opposite
side.
-It lies in tegmentum, posterior to
substantia nigra.
Caudal midbrain at level of
Inferior colliculus
8-Substantia Nigra : It is a large
T.S.of caudal midbrain at level of inferior colliculus.
extrapyramidal motor nucleus, lies at
midbrain tegmentum.
-It contains subdivision part, the pars
compacta, which consists of pigmented,
melanin-containing neurones that
synthesize dopamine as their transmitter.
-It project to caudate nucleus+putamen of
basal ganglia in the forebrain.
--It has extra-pyramidal motor function,
concerned with movements.
-lesion of pars compacta leads to
parkinson’s disease. It is due to absence
of dopamine into basal ganglia, this is
manifested by a mask face,
resting tremors, rigidity of muscles
(more in flexors giving flexor attitude) and
a shuffling gait.
Caudal midbrain at level of
Inferior colliculus
9-Crus Cerebri : lies ventral to
substantia nigra. It consists entirely of
descending cortical efferent Fs.
-50% of crus consists of pyramidal tract
consists of cortico-bulbar Fibres (end in
motor cranial nerve nuclei of brain stem) +
cortico-spinal fibres down to medullary
pyramid and spinal cord.
T.S.of caudal midbrain at level of inferior colliculus.
-on either side of corticobulbar &
corticospinal fibres, crus cerebri
contains cortico-pontine,
temporo-pontine + fronto-pontine fibres
-These Fs. arise from cerebral cortex
and ends in pontine nuclei of ventral
pons to pass via M.C.P into cerebellum,
(cortico-ponto-cerebellar pathway ) to
involve in coordination of movement.
Rostral midbrain at the level of
Superior Colliculus:Superior colliculus :
-lies in upper part of tectum of midbrain.
-it is a centre of visual reflex.
-Its main afferent Fs. are :
Cortico-tectal Fs. arise from
-visual cortex of occipital lobe.
-frontal eye field of frontal lobe.
-function : control movements of eyes +
accomodation reflex.
.
Pretectal nucleus : It lies above the
superior colliculus.
-It receives the visual Fs. running in optic
tract just rostral to superior colliculus.
-it has connections with parasymp.
nucleus of oculomotor N.
(Edinger-Westphal nucleus) to control
smooth ms. of eye (sphincter pupillae)
and to mediate pupillary light reflex.
Rostral midbrain at the level of
(central) grey.
Superior Colliculus Peri-aqueductal
:
Oculomotor nucleus : lies ventral to
peri-aqueductal grey at level of superior
colliculus of midbrain.
-efferent Fs. emerge from the medial
surface of crus cerebri. as oculomotor
nerve to supply extraocular ms. of eye
(except S.O + L.R.).
Red nucleus :
-it is a large mass of grey matter lies in
tegmentum of rostral midbrain.
-it has spinal extrapyramidal motor function.
-It receives afferents from motor cortex &
cerebellum (cortico-rubral & dentato-rubral F.).
-it sends efferents to spinal cord as rubrospinal tract.
Reticular formation
It is a gray matter extending throughout
the length of brain stem, made up of ,
deeply placed nerve cells & fibres.
It has important functions for vital
centers as respiratory & cardiovascular
centres.
It has descending fibres,
reticulospinal tracts that influence
muscle tone & posture.
It has ascending fibres, the reticular
activating system (RAS), plays a role in
consciousness and awake / sleep cycle.
Reticular formation
Raphe nuclei
Raphe nuclei : are a group of midline nuclei
that extend throughout the length of brain stem.
-they are serotonergic nuclei (their transmitter
is serotonine).
-Their ascending fibres to forebrain are involved
in neural mechanisms regulating sleep.
-Descending fibres to the spinal cord are
involved in modulation of nociceptive
mechanisms.
Locus coeruleus : is a group of pigmented
Locus coeruleus
neurones that lies in brain stem tegmentum of caudal
midbrain & rostral pons.
-it is noradrenergic cell group.
-it has ascending fibres to cerebellum,
thalamus,hypothalamus, limbic system and cerebral
cortex.
-its descending fibres project to brain stem & spinal
cord.
-involved in neural mechanisms regulating sleep.
Brain stem lesions :
 A unilateral brain stem lesion : caused
by stroke,tumour or multiple sclerosis
causes :
1-epsilateral cranial nerve dysfunction +
contralateral spastic hemiparesis.
2-hyperreflexia & an extensor plantar
response (upper motor neurone lesion).
3-contalateral hemisensory loss.
4-ipsilateral incoordination.
5-it can affect eye movements through
demyelination of medial longitudinal
fasciculus, producing internuclear
ophthalmoplegia which interferes with
conjugate ocular deviation (abducting eye
moves normally, but adducting eye fails to
follow), adduction is preserved on
convergence.
A bilateral lesion : destroys the ‘vital
centers’ for respiration & circulation, leading to
coma & death.
Examples of Questions of Midbrain :
1. Which nucleus is not lying in the tegmentum of the midbrain ?
a. Oculomotor nucleus.
b. Trochlear nucleus.
c. Mesenchephalic nucleus of trigeminal
d. Red nucleus
e. Abducent nucleus.
2.Substantia nigra is concerned with :
a.Hearing sensation.
b.Visual sensation.
c.Motor function.
d.Pain and temperature sensation.
e.Neural mechanisms regulating sleep.
3. The extrapyramidal nucleus lying in tegmentum of
the midbrain is :
a. Oculomotor nucleus.
b. Trochlear nycleus.
c. Substantia nigra.
d. Mesenchephalic nucleus of trigeminal.
e. Facial nucleus.
4.Parkinson's disease results from degeneration of:
a.Red nucleus.
b.Medial lemniscus.
c.Pyramid.
d.Substantia nigra.
e.Inferior olivary nucleus.
5. Which is wrong regarding the contents of the crus
cerebri of midbrain :
a.Corticospinal fibres.
b.Corticobulbar fibres.
c.Frontopontine fibres.
d.Temporopontine fibres.
e.Corticotectal fibres.